1. Field of the Invention
The present invention relates to an improvement in a roll used, for example, as a development roll in an electro-photographic apparatus such as a copying machine, a printer, a facsimile, etc.
2. Description of the Prior Art
Recently, a simplification of maintenance has been promoted in a field of an electro-photographic device, such as copying machine, a printer, a facsimile and so on, especially, a small electro-photographic device. In practice, a development device (hereinafter referred to as prior art) is used to develop an electrostatic latent image formed on an image bearing body such as a surface of photosensitive body, a photosensitive paper, a recording paper, etc., by using single component toner as disclosed in Japanese Patent Publication No. Sho 53 (1978)-3233.
FIG. 14 shows a conventional typical development apparatus. The development apparatus comprises a photosensitive drum 101 which includes a surface layer portion A and a substrate portion B.
The surface layer portion A acts as an image bearing body and the substrate portion B acts as a supporting member for the image bearing body.
The substrate portion B comprises a conductive elastic rubber 103 adhered on a surface of a metallic drum 102 and a flexible metallic foil 104 such as aluminum foil adhered on the rubber. The surface layer portion A comprises a photoconductive insulating layer 105 in which a metal such as selenium is formed by evaporating it on the metallic foil.
A development roll 106 is provided into press-contact with a surface of the photosensitive body 101.
The development roll 106 has a metallic roll 107 and an elastic layer 108 composed of a compositive rubber and a urethane foam, etc, formed on a surface of the metallic roll.
Because a plasticizer and a low molecular substance which have been bred out from the elastic layer 108 of the development roll 106 contaminate the surface layer portion A of the photosensitive body 101 and adhesion (tacking) and filing of the toner occur on the elastic layer 108 of the development roll 106, there has been also developed a development roll for preventing breed out of the plasticizer and low molecular substance and the adhesion and filming of the toner by coating a surface layer (not shown) of resin having a good mold-releasing property of toner on the surface of the elastic layer 108 of the development roll 106.
A bias power source 109 is connected between the development roll 106 and the substrate portion B of the photosensitive drum 101.
A hopper 111 for containing single component nonmagnetic toner 110 is provided at an upper portion of the development roll 106 in such a manner that a lower opening of the hopper is arranged with a predetermined space from the surface of the development roll 106. A frictional charging member 112 is adhered to an inside of a right side wall of the hopper. A uniformized member 113 is provided in such a manner that a surface thereof is arranged in press-contact with the surface of the development roll 106.
The uniformized member 113 comprise a metallic roll 114, a rubber layer 115 adhered to a surface of the roll and a frictional charging member 116 coated on the rubber layer. The uniformized member is not rotated.
According to the development roll 106, a thin layer of toner is carried onto the surface of the development roll 106. The roll formed in such way is abutted with the photoconductive drum 101 to which electrostatic latent image is formed and then the toner is transferred from the development roll 106 to the photoconductive drum 101 in response to development electric field to visualize the electrostatic latent image recorded on the photoconductive drum 101.
In the aforementioned development roll, it is necessary to control polarity of charge and charging amount of the toner by fictional charge due to contact between the toner and the surface of the development roll 106.
The transfer of toner to the photoconductive drum 101 is performed in response to the development electric field and the polarity of toner charge by selecting an image portion of the electrostatic latent image or a non-image portion (naked portion) of the latent image. According to this apparatus, there is an advantageous effect that colorization is easy without using any magnetic material for the toner.
In the conventional development apparatus using single component of toner, some attempts are made to reduce hazards upon charging by lowering charging potential in order to lengthen life duration of the apparatus, especially the long life duration of the photosensitive body. Thus, there has been needed a system capable of developing with a reduced development potential.
From the viewpoint of higher image quality, it has been required to make as low as possible the saturated development potential and to sharpen the development xcex3 (gradation of a characteristic curve for image concentration level with respective to the electrostatic potential) to a possible extent. This enables to change from multi-values which provide a writing modulation pattern of image by using a great number of gradations, to binary values which provide the writing modulation pattern of image by using two gradations (i.e., perform change of gradation by operation of dots with the same concentration of dots), in order to change a gradient expression of image in digital to an area gradation.
Reducing resistance of the development roll to sharpen the developments xcex3 is a well-known. However, if the resistance of the development roll is reduced, a higher voltage, e.g., xe2x88x92250 V as the bias voltage is applied. Accordingly, there is a problem that a leak of electricity to the electrostatic latent image bearing body occurs. For this reason, in prior art, the leak of electricity to the electrostatic latent image bearing body has been prevented by adding conductivity-imparting agent such as carbon black to the elastic layer 108 of the development roll 106 thereby to make the resistance of the elastic layer 108 lower than that of the surface layer of the development roll 106.
However, if the conductivity-imparting agent such as carbon black is highly charged in the elastic layer 108, since the elastic layer is hardened, there are problems that (1) a sufficient nip cannot be obtained between the development roll 106 and the photoconductor drum 101, thus resulting in a deterioration of image quality and (2) manufacturing facility is impaired, thus resulting in a difficulty in forming the elastic layer 108 into a shape of roll.
It is an object of the present invention to provide a roll for resolving the aforementioned problems in prior art. More specifically, it is an object of the present invention to provide a roll capable of carrying out reductions in hardness and resistance without any fear of leak to the latent image, and provide a development apparatus for using the roll.
To accomplish the above object, according to an aspect of the present invention, a roll is provided in which it comprises a core shaft, an elastic layer arranged around said core shaft and having a resistance, and a surface layer arranged around said elastic layer and having a resistance. The resistance of the surface layer is smaller than that of the elastic layer so that the entire volume resistance of the roll is less than a volume resistance of the elastic layer.
In one embodiment, the volume resistance of the elastic layer is less than 1.0xc3x97109 xcexa9xc2x7cm. The entire volume resistance of the roll is also less than 1.0xc3x97107 xcexa9xc2x7cm.
Further, an actual resistance of said surface layer is less than 1.0xc3x97108 xcexa9, and a thickness of said surface layer is less than 30 xcexcm.
According to the other aspect of the present invention, a development apparatus is provided, in which it comprises the roll as described above.
In one embodiment, the development apparatus has a photosensitive body having an image portion and a quantity of toner adhering onto said photosensitive body is saturated under a condition that a difference between a development bias and a surface potential of said image portion in said photosensitive body is less than 150 V.